Occurrence, density and distribution of the parasitic fungus Hesperomyces virescens Thaxter (Laboulbeniales: Laboulbeniaceae) on the multicolored Asian lady beetle (Coleoptera: Coccinellidae)

Authors: Riddick, Eric; Schaefer, Paul

Submitted to: Annals of the Entomological Society of America
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/17/2005
Publication Date: 7/3/2005
Citation: Riddick, E.W., and Schaefer, P.W. Occurrence, density and distribution of the parasitic fungus Hesperomyces virescens Thaxter (Laboulbeniales: Laboulbeniaceae) on the multicolored Asian lady beetle (Coleoptera: Coccinellidae). Annals of the Entomological Society of America. 98(4):615-624. 2005.

Interpretive Summary: The multicolored Asian lady beetle has received much attention in recent years for its usefulness as a natural enemy of soft-bodied insects (such as aphids) in various crops in North America and for its annoying behavior of invading houses during the fall and winter seasons. Adults of this lady beetle were found to be infected with a fungus disease while overwintering in a observation tower in Pennsylvania, USA. It was demonstrated that the fungus has good potential to spread to uninfected beetles under artificial conditions in the laboratory. In the field, infection rates were much less than that seen in the laboratory. This research is relevant to ecologists, mycologists and entomologists in industry, government and academic institutions.

Technical Abstract: A coccinellid-specific parasitic fungus, Hesperomyces virescens Thaxter, was found on adults of the multicolored Asian lady beetle, Harmonia axyridis (Pallas), in fall and winter seasons in Lebanon Co., Pennsylvania. Research objectives were (1) to determine the density of H. virescens on field-collected H. axyridis adults held under artificial conditions in the laboratory, and (2) to determine H. virescens presence, density and distribution on H. axyridis adults in the field. Hesperomyces virescens was serendipitously discovered on the integument of H. axyridis adults, which had been removed from an overwintering site (on November 2002) and cold-stored in a laboratory refrigerator (at 5°C). As a result of removing beetles from cold-storage and holding them under crowded conditions (at 22°C) for 28 d, 100% of H. axyridis adults contained H. virescens thalli (i. e., fruiting bodies). Male and female H. axyridis hosted an average of 199 and 169 H. virescens mature thalli, respectively; distributed primarily on the elytra and the abdomen. At the overwintering site, H. virescens density per host was often less than 20 mature thalli, which were distributed primarily on the elytra of both sexes. On average, 52.5 and 57.4% of H. axyridis males and females, respectively, hosted H. virescens mature thalli in late winter (5 March 2003). Less than 14% of H. axyridis adults hosted mature thalli of H. virescens the following fall (15, 22, 28 October and 10 November 2003) at the same site. Collection of infected H. axyridis suggests that this lady beetle is a host for H. virescens in Pennsylvania and crowded conditions in laboratory cages can exacerbate the spread of H. virescens ascospores via physical contact between conspecifics. Preponderance of fungal thalli on the dorsum rather than the ventrum of H. axyridis males does not conform to the sexual transmission hypothesis. Field estimates of infection may vary considerably between dates that H. axyridis adults arrive and depart from overwintering sites.